Aqueous polyurethane dispersions are well known and are used in the production of a variety of useful polyurethane products, including, for example, coatings, films, adhesives, sealants, and the like, collectively referred to as so-called "ACS". Such dispersions are typically produced by dispersing a water-dispersible, isocyanate-terminated polyurethane prepolymer in an aqueous medium together with an active hydrogen containing chain extender, such as a diamine.
The prepolymers used in the preparation of the dispersions are generally substantially linear, that is to say difunctional, and are typically obtained by reacting an excess of a diisocyanate with an isocyanate-reactive component comprising a polymeric diol in the presence of a reactive compound, for example a diol or diisocyanate, containing an ionic or nonionic hydrophilic group or site.
Attempts to introduce higher functionality into the dispersed polyurethanes, in an effort to enhance the physical properties of the resulting ACS or shorten the curing time for these products, have not been entirely successful. One method of increasing the functionality is by incorporating a higher functionality polyol, such as a triol or tetrol, into the prepolymer, and the use of such polyfunctional active hydrogen compounds in the preparation of ionic polyurethane dispersions has been described in U.S. Pat. No. 4,554,308. The amount of polyfunctionality that can be introduced in this way is limited by the tendency of the more highly crosslinked prepolymers to gel and to form large micelles when dispersed, resulting in poor film-forming properties for this dispersion.
Another method of introducing higher functionality employs a linear prepolymer in conjunction with a trifunctional chain extender, such as diethylene triamine. This approach has been described in U.S. Pat. Nos. 4,203,883 and 4,408,008. One problem associated with these triamine cross-linked urethane dispersions is that they tend to form gels prior to use and they tend to provide poor coalescence of the films on substrates during use.
Yet another method of introducing higher functionality uses a triisocyanate in the prepolymer preparation, but this methodology causes similar problems to those that result when using a triol or tetrol in the preparation of the prepolymer, including gelation and poor dispersion of the prepolymer due to large micelle formation.
A different method of incorporating triisocyanates has been described in U.S. Pat. No. 4,507,431 which describes a process for preparing aqueous dispersions of cross-linked polyurethane ionomers. These dispersions are made by mixing an isocyanate containing prepolymer dissolved in a water-miscible organic solvent having a boiling point of from 20 to 100.degree. C. with a polyfunctional polyisocyanate cross-linking compound having an isocyanate functionality of about 2.2 to 4. The isocyanate-containing prepolymer is prepared from a linear polyhydroxy compound having a molecular weight of from 800 to 5000, and the prepolymer has exclusively aliphatic or cycloaliphatic terminal isocyanate groups in amounts from 0.1 to 2% by weight incorporated therein by employing an aliphatic diisocyanate and/or cycloaliphatic diisocyanate compound alone as the diisocyanate reactant with said polyhydroxy compound or in conjunction with a non-aliphatic or non-cycloaliphatic diisocyanate reactant, and containing salt groups in amounts of from 0.02 to 1% by weight; so that there are from 0.1 to 1.5 isocyanate groups of said polyisocyanate per isocyanate group of said prepolymer; dispersing the resulting solution in from 40 to 80% by weight, based on the polyurethane prepolymer and the polyisocyanate, of water and evaporating the organic solvent. Unfortunately, evaporation of the solvent in the production of the aqueous dispersion in accordance with the teachings of the '431 patent is undesirable from an environmental standpoint.
Still another method of preparing aqueous polyurethane dispersions is disclosed in U.S. Pat. No. 5,169,895. The '895 patent discloses an aqueous dispersion of a water-dispersible polyurethane, wherein the polyurethane represents the reaction product of (a) a water-dispersible isocyanate-terminated polyurethane prepolymer having an NCO content of 2.1 to 10% by weight, (b) an organic polyisocyanate having an average isocyanate functionality of 2.1 to 4.0, and (c) an active hydrogen-containing chain extender, but with the proviso that the polyurethane is prepared in the absence of the steps recited to prepare the polyurethane dispersion in the above-described '431 patent. Examples of the organic polyisocyanate of component (b) of the '895 patent include the trimers of hexamethylene diisocyanate (so-called "HDI"), isophorone diisocyanate (so-called "IPDI"), and 2,4-toluene diisocyanate, biuret-modified HDI, and the adduct of 2,4-toluene diisocyanate and trimethylolpropane, as recited at column 5, lines 27-31 of the '895 patent.
New aqueous dispersions of polyurethanes providing improved physical properties for ACS applications, relative to the above-described prior art compositions such as those described in the '895 patent, would be highly desired by the ACS community. The present invention provides one such improved composition.